It is known that, during a cold start of an internal combustion engine equipped with fuel injectors, the first few combustion cycles contribute a significant amount of hydrocarbons during an emissions test cycle. It is also known that if fuel is heated before it exits the tip or barrel end of a fuel injector, atomization of fuel is improved through smaller droplet size. This improvement in atomization allows for more complete combustion, which results in lower emissions and increased fuel economy.
In order for the initial pulses of fuel from the fuel injector to be heated, the heat source must be able to heat the fuel that resides just upstream of the metering valve in the barrel end of the injector. In one prior art example, it is known to cover the outside of a fuel injector barrel over a portion of its circumferential surface with a thick film resistance heating element. However, in the known art, a measurable gap along the adjacent axial edges of the heating element must be maintained to electrically insulate the opposing poles of the heating element from each other. Therefore, only about 65% of the surface area of the fuel injector barrel may be heated directly by the resistance heating element.
In another prior art example, a resistance heating element formed in a long, narrow strip is wrapped around a fuel injector barrel in a helical path. The connector pads are then bonded to each end of the helix. In this design, since each loop of the helix must be spaced from the adjacent helix loop in order to assure a current flow path through the entire helix, and since the connector pads consume a fair length of the heating element at each end of the helix, the surface area of the fuel injector barrel contacted by the active portion of the heating element is significantly reduced as well.
These arrangements have at least three shortcomings.
First, because the heating element does not come in direct contact with a substantial amount of the fuel injector barrel surface, the fuel injector barrel has non-heated areas. Thus, fuel therewithin is heated non-uniformly. To overcome this, it is known to provide a static mixing element within the barrel to channel cold fuel circumferentially into the heated region during the flow of fuel axially through the barrel and to mix the cold fuel with heated fuel. This solution provides only a marginal improvement and adds significant cost, complexity, and bulk to a fuel injector with this design.
Second, the resistance element typically is applied in a single “thick” coating and for various reasons a typical coating may vary in thickness, and consequent resistance, by about 20%. In order to reduce areal variability in heating, it is known to trim thick film heaters by laser, by partially cutting into the surface of the resistance element in selected locations. However, the cuts into the surface weaken the integrity of the heater film, with possible cracking, and provide points where contamination may be collected, either or both potentially causing heater element failure.
Third, depending upon the fuel injector's heater design, relatively small hot spots can occur in the resistance heater, as for example, near cut-outs or islands, which are necessarily provided on the surface of the resistance element or where connector pads attach to the resistance element. These spots result in decreased and non-uniform heat transfer to the fuel. It has been found that these hot spots can be reduced by selectively adding one or more localized layers of resistance coating to the resistance layer.
What is needed in the art is a fuel injector having a thick resistance element on the outside of the barrel wherein coverage of the barrel by the resistance element is optimized and wherein resistance is uniform to within about 5%.
It is a principal object of the present invention to improve the uniformity of fuel heating during passage of fuel through a fuel injector barrel.